This was due to.....

The gun was loaded with plastic bullets that would make a splat when they hit something. They were also capable of breaking something made of glass.

The gun was fired and the last round did not have enough power to clear the barrel. So there was a wad of plastic stuck in the barrel.

The scene with Lee was using blanks, but the barrel was not checked for obstructions. When the blanks was fired the stuck plastic round was discharge and that struck Lee and killed him.

This was an accident, but it could have been prevented. Making a film is like any other industrial process: Safety First.